Method for manufacturing an integrated circuit with programmable non-overlapping-clock-edge capability

ABSTRACT

A system and method for generating and optimizing clock signals with non-overlapping edges on a chip using a unique programmable on-chip clock generator. Overlapping of the edges of the clocking signals is avoided by adjusting an amount of delay introduced in the on-chip clock generator circuit. The amount of delay is adjusted by programming the on-chip clock generator using either hardware and/or software programming. In hardware programming, the amount of delay adjusted by physically altering the composition of delay elements in the on-chip clock generator. In software programming, the delay is adjusted using software commands to control the operation of delay elements in the on-chip clock generator, or to select the paths that delay the signals.

This application is a continuation of application Ser. No. 08/478,534,filed Jun. 7, 1995, now abandoned, which is a division of applicationSer. No. 08/255,910, filed Jun. 8, 1994, U.S. Pat. No. 5,444,405, whichis a continuation of application Ser. No. 07/967,614, filed Oct. 28,1992, now abandoned, which is a continuation-in-part of application Ser.No. 07/844,066, filed Mar. 2, 1992, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to a programmable clockgenerator for integrated circuits, very large scale integrated circuits(VLSI), and ultra large scale integrated circuits (ULSI). Moreparticularly, the present invention relates to a programmable clockgenerator for selecting optimal non-overlapping clocking signals forcontrolling elements on a chip.

2. Related Art

Most microprocessor chips operate as control driven, synchronoussequential systems. This means the sequence of operations in the systemis synchronized by a master clock signal (usually an external clock).This clock signal is usually one of the forms shown in FIG. 1; whichillustrates a square wave with a 50% duty cycle.

The master clock signal allows system operations to occur at regularlyspaced-intervals. In particular, operations on the chip are made to takeplace at times when the clock signal is making a transition fromlow-to-high or from high-to-low; rising edge 102 or falling edge 104,respectively.

Many microprocessor chips have their timing controlled by two or morerelated clock signals generated by an on-chip clock generator based onthe master clock signal. FIG. 2A illustrates one such combinationutilizing two clock signals identified by φ1 and φ2. This clockingarrangement provides four different edges and three different states perperiod, compared to only two edges and two states per period providedwith a single clock signal as shown in FIG. 1. FIG. 2B illustratesexamples of the three possible states for clock signals φ1 and φ2. Forelements on the chip to function properly, it is important that edges ofclock signals φ1 and φ2 are non-overlapping. If the edges overlap therewill be more restrictions on data transfer and signal hand shaking.

Additionally, it is equally important that non-overlapping clock edgesbe evenly distributed to all corners of a chip regardless of thedistance which those signals must travel. As chip size increases, clocksignals φ1 and φ2 have to travel greater distances throughout the chip.This causes clock signals φ1 and φ2 to become degraded. As distancesincrease, rising edges 202, 206 and falling edges 204, 208 may becomeobscured (experience phase shifts and increases in transition times) andcan overlap. This phenomenon, sometimes referred to as clock skew, iscaused by a number of factors, including: loading, unwanted noise,coupling, capacitance, resistance, inductance and other debilitatingeffects.

To account for these factors, designers must separate the rising andfalling edges 202, 204, 206, 208 of different clock signals (i.e., φ1and φ2) with a large enough margin of time to allow for clock skew. Forinstance, falling edge 204 and rising edge 206 must be separated by aminimum temporal distance or amount of time (T) to avoid overlappingstates; especially for level-triggering operations inmetal-oxide-silicon (MOS) technology. The larger T is, the less likelythe chip will fail due to overlapping signals caused by skewing. Thewide range of operating environments to which the chip(s) may be subjectmust be considered in selecting T. Therefore, to provide an adequatemargin, manufacturers are forced to select T large enough to providefunctionality in a worst-case environment. However, a large T is asignificant cycle time constraint. Therefore chip design is notoptimized for each environment.

To illustrate this, consider current chip design practices that mustaccount for clock skew by designing a chip with a minimum safetydistance T between signals against worst-case conditions. Once T isselected the chip is manufactured and tested. If the chip designerselected a clock speed that has insufficient non-overlapping time, thechip will not function due to overlapping states for some circuitslocated on the chip. When a chip runs properly, chip designers assumethey have chosen the correct frequency, clock states, rise and falltimes, and non-overlapping time T. However, chip designers do not knowwhether a faster clock speed or a smaller T are possible. To find out,chip manufacturers must build entirely new chips with different processparameters, which is inefficient and expensive.

Presently, no programming or tweaking can be performed after a chip isfinalized. It is possible to have an on-chip clock generator running atdifferent clock frequencies than external crystal oscillators, but thenon-overlapping time of the clock edges generated by the clock generatoris fixed by circuit hardware. Therefore, what is needed is a flexiblesystem and method of programming an on-chip clock generator at themanufacturing stage to achieve adjustable as well as optimalnon-overlapping times T between clock edges.

At the post-manufacturing stage, environmental conditions, such as heatand cold can also affect clock skewing. If a chip is manufactured underlaboratory conditions, it may function properly. However, temperaturechanges may cause the chip to malfunction due to skewed clock signals.Therefore, what is needed is an on-chip clock generator that can bedynamically programmed to select non-overlapping times T to account forenvironmental fluctuations while the chip is in an operationalenvironment, such as a processor chip operating in a computer.

SUMMARY OF THE INVENTION

The present invention is directed to a system and method for providingprogrammable non-overlapping clock generation on a chip. The presentinvention includes four main embodiments. The first embodiment isdirected to the overall operation of an on-chip clock generator. Thesecond embodiment is directed to a hardware programmable clockgeneration system and method. The third embodiment is directed to asoftware programmable clock generation system and method. The fourthembodiment is directed to a combination of all three embodiments.

The programmable on-chip clock generator provides two phases of a systemclock with non-overlapping edges. The programmability of the clockgenerator provides flexibility during chip fabrication, and when a chipis functioning in a operational environment.

During the manufacturing phases of chip production, characteristics ofthe on-chip clock generator are altered to ensure the edges of the twogenerated clocks do not overlap. This allows the manufacturer tooptimize the performance of the chip while the chip is undergoinginitial production testing. This feature obviates the need to performcostly and time consuming trial-and-error design and redesign of on-chipclock generators.

Additionally, the present invention provides a technique for optimizingthe performance of the on-chip clock generator after the chips have leftthe manufacturing environment. One feature of the present invention isthe ability to adjust clock generation dynamically to account forclimatic changes in an operational, or other post-production,environment. This allows chips to be manufactured with wider tolerancesand allows operation of the chip to be optimized when the chip is in theoperational environment.

Adjustments to the on-chip clock generator during the manufacturingphase are referred to as hardware programming because the manufactureralters the physical composition of the clock generator. Adjustments tothe on-chip clock generator once the chip is fabricated and in theoperational environment are referred to as software programming. Thisterminology reflects the fact that through the use of software commands,the characteristics of the on-chip clock generator can be adjusted tocompensate for changes in the operating environment. Programmingcapability in both cases is accomplished by adding or subtracting delayelements in feedback paths within the clock generator circuit.

Further features and advantages of the present invention, as well as thestructure and operation of various embodiments of the present invention,are described in detail below with reference to the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a square wave with a 50% duty cycle.

FIG. 2A illustrates two clock signals identified by φ1 and φ2 withnon-overlapping edges.

FIG. 2B illustrates three possible states for clock signals φ1 and φ2.

FIG. 3 illustrates a high-level block diagram of an environment in whichthe present invention operates.

FIG. 4 illustrates a high-level block diagram of clock generatoraccording to the present invention.

FIG. 5 is a flow chart illustrating the operation of a clock generatoraccording to the present invention.

FIG. 6A illustrates a circuit diagram of a clock-to-clocknon-overlapping time adjuster according to a hardware embodiment of thepresent invention.

FIG. 6B illustrates a logic circuit diagram of a clock-to-clocknon-overlapping time adjuster with generic delay paths.

FIG. 6C illustrates a timing diagram of the operation of theclock-to-clock non-overlapping time adjuster.

FIG. 7 illustrates an example of hardware programming according to thepresent invention.

FIG. 8A illustrates a circuit diagram of a clock-to-clocknon-overlapping time adjuster according to a software embodiment of thepresent invention utilizing path selection to adjust delay.

FIG. 8B illustrates a circuit diagram of a clock-to-clocknon-overlapping time adjuster according to a software embodiment of thepresent invention utilizing path length selection to adjust delay.

FIG. 9 illustrates an example combination hardware and softwareembodiment of a clock to clock non-overlapping time adjuster system,according to the present invention.

In the drawings the left-most digit of a reference number identifies thedrawing in which the reference number first appears.

DETAILED DESCRIPTION OF THE INVENTION I. General Overview

A. Brief Overview

The present invention is directed to a system and method for providingnon-overlapped clock generation on a chip. The present inventionincludes four main embodiments. The first embodiment is directed to theoverall operation of an on chip clock generator. The second embodimentis directed to a hardware programmable clock generation system andmethod. The third embodiment is directed to a software programmableclock generation system and method. The fourth embodiment is directed toa combination of the first three embodiments. These embodiments of thepresent invention are discussed in the following sections.

B. Environment

FIG. 3 illustrates a high level block diagram of an environment 301 inwhich the present invention operates. Environment 301 may be a wafercontaining hundreds of chips at a fabrication/testing stage or acomputer in a user environment. As illustrated, environment 301 includesan external clock generator 302 and a chip 304. In a preferredembodiment, external clock generator 302 is a crystal oscillator, whichproduces a signal similar to that shown in FIG. 1.

Chip 304 has circuit elements 310. Many computers require more than oneprocessor chip. As shown in FIG. 3, an optional co-processor orperipheral chip 305 may be coupled to chip 304.

Chip 304 also has an internal clock generator 308, which providesmultiple clock signals to elements 310. In a preferred embodiment clockgenerator 308 provides two clock signals, φ1 and φ2; similar to FIG. 2A.To ensure that active portions of clock signals φ1 and φ2 arenon-overlapping, clock generator 308 is programmed to achieve optimalnon-overlapping clock generation. Accordingly, clock generator 308provides a plurality of programmable non-overlapping times between clocksignals φ1 and φ2. In the preferred embodiment, clock generator 308provides the option of selecting between 0.5 ns, 2.5 ns and 4.5 ns ofnon-overlapping time between clock signals φ1 and φ2 shown as in FIG.2A. In alternative embodiments, T must be less than the period of CLKIN401. Furthermore, for a 50% duty cycle T should be less than the periodof CLKIN/2. If this condition is not met, the circuit may chop outclocks.

This flexibility permits adjustments to clock generation if there is notenough holding time between clock edges by increasing T between clocksignals φ1 and φ2. On the other hand, if there is too much holding timebetween clock signals φ1 and φ2, T can be decreased. In either case,"tweaking" can be performed while the chip is being tested (during themanufacture stage) without expensive and costly changes to the clockdesign.

Adjustments to clock generator 308 can also be performed in anoperational environment (product usage stage). As a result, clockgenerator 308 can be adjusted for climatic changes which can affectwhether clock signals φ1 and φ2 are non-overlapping.

II. Clock Generator

FIG. 4 illustrates a high level block diagram of clock generator 308.Clock generator 308 includes an input waveform stabilizer 402, aclock-to-clock non-overlapping time adjuster 406, and a clock driver 410having a two phase output φ1 and φ2 (CLK OUT 411).

Operation of clock generator 308 is generally illustrated in the flowchart shown in FIG. 5. In describing the operation of clock generator308 reference will be made to FIGS. 2-8.

Referring now to FIGS. 4 and 5, in a step 502, clock generator 308receives an external clock signal 401 from external clock 302 shown as"CLKIN" 401 (as shown in FIG. 4). In a step 504, input waveformstabilizer 402 stabilizes CLKIN 401. Waveform stabilizer 402 reshapesCLKIN 401 into a square wave (CLK 405) because CLKIN 401 tends to bedistorted due to input jitter, noise from ground bounce and coupling. Inthe preferred embodiment, waveform stabilizer 402 is a Schmitt trigger.The structure and operation of a Schmitt trigger are well known to thoseskilled in the art.

A. Programming

In a step 508, CLK 405 is received by clock-to-clock non-overlappingtime adjuster 406, and multiple signals with non-overlapping activephases 407 are generated with adjustable delay between edges. The clockgenerator 308 can be programmed to provide desired holding times betweenclock edges.

There are two types of programming that can occur: hardware andsoftware. Hardware programming normally occurs during the testing stagesof a chip and involves altering the chip physically. This process istypically irreversible. Software programming normally occurs while achip is functioning in an operational environment. Software programmingis dynamic, allowing adjustments to delay time T between clock edgeswithout physically altering the chip. Programming capability in bothcases is accomplished by adding or subtracting delay elements infeedback paths within the clock generator circuit (to be described).

1. Hardware Programming

FIG. 6A illustrates the hardware embodiment of a clock to clocknon-overlapping time adjuster 406 according to the present invention.Referring to FIG. 6A, clock-to-clock non-overlapping time adjuster 406includes logic gates 602, 603, delay element(s) 604 and drivers 622,624.

In the preferred embodiment, logic gates 602 and 603 are NOR gates anddelay elements 604 are inverters. Other logic elements can besubstituted for the ones described in the preferred embodiment. Forinstance, inverter 604 can be replaced with NAND gates having inputstied together. Additionally, delay elements 604 can be any device (i.e.resistors) that delay a signal.

Delay elements 604, form delay paths 606A, 606B (generally 606) whichgovern the amount of delay between clock signal φ1 and clock signal φ2.Adjusting the length of delay paths 606 by adding or subtracting delayelements 604 coupled to logic gates 602, 603, increases or decreases theamount of time T between edges of clock signals φ1 and φ2. Typically, achip designer will include more delay elements 604 than needed to affordlatitude in the selection of possible delay times. Selection from aplurality of pre-planned delay paths 606 provide a correspondingplurality of non-overlapping optional times between clock edges.

FIG. 6B illustrates clock-to-clock non-overlapping time adjuster 406with generic delay paths or delay segments τ1 and τ2 to delay clocksignals φ1 and φ2. Delay segments τ1 and τ2 are equivalent to delaypaths 606 shown in FIG. 6A. Changing the amount of delay allows theamount of time between edges of φ1 and φ2 to be adjusted. The amount ofdelay introduced in these segments τ1 and τ2 is programmable by addingor deleting delay elements 604.

Referring to FIG. 6B, time adjuster 406 operates as follows. An inputclock signal (CLK) 405 passes through inverter 609 to form invertedclock signal 601 (NOTCLK 601). CLK 405 and NOTCLK 601 are used to formtwo new clock signals φ1 and φ2. CLK 405 is an input signal to NOR gate602 along with a delayed signal φ2. NOTCLK 601 is an input signal to NORgate 603 along with delayed φ1 signal.

FIG. 6C is a timing diagram illustrating the operation of the clock toclock non-overlapping time adjuster 406 of the present invention.Referring to FIGS. 6B and 6C, the technique of the clock to clocknon-overlapping time adjuster 406 will now be described. Note, gatedelays, which are shown as t in FIG. 6C, are negligible. Note: t=gatedelay; τ=delay of a delay element; and T=t+τ.

As shown in FIG. 6C, in the beginning of region 1, clock 405 transitionsto a logic high state, which forces φ1 to the logic low state. A delaytime τ1 later, delayed φ1 transitions to a low state. Since NOTCLK 601is at a logic low state, this transition forces φ2 to transition to alogic high state.

As a result of delay τ1, the rising transition of φ2 lags behind thefalling transition of φ1 by the amount of τ1 plus any gate delay timest. The amount of time separating the falling transitions of φ1 from therising transitions of φ2 is controlled by adjusting τ1. If the circuitdesigner wishes to increase the clock frequency, τ1 is programmed to alesser amount. On the other hand, if the circuit operation is hamperedby overlapping falling and rising transitions of φ1 and φ2,respectively, τ1 is increased until this problem is rectified. In thismanner, the circuit is optimized by increasing the clock frequency tothe maximum permissible level without the transitions overlapping.

In a similar fashion, the falling edges of φ2 are separated from therising edges of φ1 by controlling the amount of delay programmed intoτ2. If the delay in τ2 is increased, the separation between the fallingedges of φ2 and the rising edges of φ1 is increased, and if the delay inτ2 is decreased, the separation is decreased. Therefore, the circuit canbe further optimized by adjusting the time T between the falling edgesof φ2 and the rising edges of φ1.

An example of hardware programming is illustrated in FIG. 7. FIG. 7represents a portion of delay path 606 of FIG. 6A. Programming isaccomplished by closing or opening switches 707, 711. When switch 707 isclosed, nodes 720 and 722 are shorted together. Additionally, by openingswitch 711 delay elements 604A, 604B are completely dropped out of thecircuit or "shorted out". Thus, closing switch 707 and opening switch711, bypasses delay elements 604A, 604B and decreases the amount of pathdelay for a signal. Opening switch 707 and closing switch 711 increasesthe amount of path delay for a signal that passes through delay path606.

There are a number of ways to close or open switches 707, 711. Suchtechniques include fuse/anti-fuse processing, laser burning, ion beammilling and other techniques. In the preferred embodiment laser "zap"burning technique is used to open/close switches 707, 711.

2. Software Programming

FIGS. 8A and 8B are logic level circuit diagrams of a clock-to-clocknon-overlapping time adjuster 406 according to a software embodiment ofthe present invention. FIGS. 8A and 8B are similar to FIG. 6 with theexception of the manner in which the delay elements τ are implemented.In other words, the software embodiment operates in a manner similar tothat of the hardware embodiment described above. However, in thesoftware embodiment, the delay times τ1 and τ2 are not fixed at the timeof fabrication as they are in the hardware embodiment. In the softwareembodiment, the amount of delay chosen for τ1 and τ2 is selected usingsoftware, and can be changed as required to compensate for changes inoperating conditions or other operational parameters.

FIG. 8A illustrates a software embodiment using path selection to adjustdelay times τ1 and τ2. Referring to FIG. 8A, multiple paths (generally822) are established on the chip, each having different propagationdelay times. The paths 822 are duplicated for τ1 and τ2. Delay paths 822can be implemented using a multiplicity of delay elements. FIG. 8A showsdelay paths 822 made up of inverters 604. If inverters are chosen, aneven number must be used in each path 822 for proper operation.

The manner of path selection will now be described. Control words 850A,850B are generated and sent to demultiplexers 824A, 824B selecting thepaths to be followed. Control words 850A, 850B are also sent tomultiplexers 825A, 825B for selecting paths to be followed. Controlwords 850A, 850B command demultiplexers 824A, 824B and multiplexers825A, 825B to route the signals output from NOR-gates 826A, 826B,respectively, through a specified path 822. The amount of delay,therefore, varies depending on the propagation delay of the selectedpath 822.

FIG. 8B illustrates a software embodiment using path-length selection toadjust delay times τ1 and τ2. In this embodiment paths 842A, 842B aremade up of strings of delay elements 846 and multiple connection points848. Control words 850 are sent to switches 844A, 844B. The controlwords 850A, 850B `command` switch 844A, 844B, respectively to select thesignal at one of connection points 848 along the string. The fartheralong the string connection point 848 is chosen, the longer the delaytime will be. Switches 844A, 844B are multiplexers.

Strings 842A, 842B are shown in FIG. 8B as being implemented usinginverters as the delay elements 846. If inverters are used, the pathsmust be chosen such that only an even number of inverters can beselected to make up the delay of the signal. A number of other elements846 can also be chosen to make up delay paths 842A, 842B.

B. Combination Embodiment

The present invention can be implemented using a combination of thehardware and software embodiments described above. For example, thedelay strings can be first adjusted to a maximum delay time as describedin the hardware embodiment. Then, fine tuning can be accomplished in theoperational environment using the software embodiments described above.

FIG. 9 illustrates an exemplary clock to clock non-overlapping timeadjuster system 902, wherein delay is determined or controlled through acombination of hardware and software. The system 902 is similar to thesystems illustrated in FIGS. 6A, 6B, 8A and 8B, with the exception ofthe manner in which the delay elements τ of FIG. 6B are implemented.Specifically, in system 902, the delay time τ1 and τ2 are partiallyfixed in hardware at the time of fabrication, and can be further refinedusing software.

In the example of FIG. 9, system 902 includes delay paths 904A, 904B,which are analogous to the delay paths 606A, 606B in FIG. 6A. One ormore of the delay paths 904A, 904B include a plurality of delay lines906, which are analogous to the paths 822 in FIG. 8A. A desired delayline 906 is selected for a path 904 through software controlled delayline selectors, illustrated here as multiplexers 925A, 925B anddemultiplexers 924A, 924B.

Operation of multiplexers 925A, 925B and to demultiplexers 924A, 924B isnow described. Control words 950A, 950B are generated and sent tomultiplexers 925A, 925B and to demultiplexers 924A, 924B. Control words950A, 950B command multiplexers 925A, 925B and demultiplexers 924A, 924Bto route signals that are output from NOR-gates 926A, 926B,respectively, through specified delay lines 906. The amount of delayvaries depending on the propagation delay of the selected delay line906, which is described below.

One or more of the delay lines 906 can be implemented as illustrated inFIGS. 6A and 7, for example. In the example of FIG. 9, the delay lines906 include one or more delay elements 908 and one or more bypasscircuits 910. The bypass circuits 910 are implemented in hardware arecan be used to bypass one or more of the delay elements 908. When adelay element 908 is bypassed, the amount of delay through theassociated delay line 906 is reduced.

In the illustrated embodiment, a separate bypass circuit 910 isassociated with each delay element 908. In an embodiment, the amount ofdelay through one or more of the delay lines 906 is established at thetime of manufacture. Preferably, when a delay path 904 includes morethan one delay line 906, each delay line 906 is set to a different delaytime to provide a range of selectable delay times for a given delay path904.

While various embodiments of the present invention have been describedabove, it should be understood that they have been presented by way ofexample only, and not limitation. Thus, the breadth and scope of thepresent invention should not be limited by any of the above-describedexemplary embodiments, but should be defined only in accordance with thefollowing claims and their equivalents.

What is claimed is:
 1. A programmable clock generator system forgenerating a plurality of clock signals having non-overlapping edges,comprising:a first delay path configured to receive a first clocksignal, said first delay path including a plurality of delay lines, eachof said plurality of delay lines including one or more delay elementsand one or more bypass circuits, wherein an amount of delay through eachof said plurality of delay lines is determined by said delay elementsand said bypass circuits, said first delay path including a softwarecontrolled delay line selector that directs the first clock signalthrough a selected one of said plurality of delay lines; and a seconddelay path configured to receive a second clock signal, said seconddelay path configured similar to said first delay path.
 2. Theprogrammable clock generator system according to claim 1, wherein saidsoftware controlled delay line selector comprises:a software controlleddemultiplexer coupled to an input of each of said plurality of delaylines; and a software controlled multiplexer coupled to an output ofeach of said plurality of delay lines.
 3. A method for adjusting anon-overlapping edge time between a first and second clock signalutilizing a programmable clock generator, the programmable clockgenerator including first and second delay paths configured to receivethe first and second clock signal, respectively, each of the first andsecond delay paths including a plurality of delay lines, each of theplurality of delay lines including one or more delay elements and one ormore bypass circuits, the method comprising the steps of:(1)establishing a delay through one or more of the plurality of delay linesusing the one or more delay elements and the one or more bypasscircuits; and (2) selecting a delay line for each of the first andsecond delay paths through software.
 4. The method according to claim 3,wherein step (1) comprises the steps of:(a) closing or opening one ormore of the bypass circuits to bypass one or more of the delay elements.